1. Field of the Invention
This invention generally relates to a semiconductor chip package and the method for making the same.
2. Description of the Related Art
Integrated circuit (chip) packaging technology is becoming a limiting factor for the development in packaged integrated circuits of higher performance. Semiconductor package designers are struggling to keep pace with the increase in pin count, size limitations, low profile mounting constraints, and other evolving requirements for packaging and mounting integrated circuits. Currently, the common packaging and package mounting technology is a ball grid array (BGA) package, in which an array of solder pads for solder balls are formed on the integrated circuit package. Conductive leads or traces on the package connect the solder pads to wire bonding pads for electrically connection to the integrated circuit chip within the package. A corresponding array of solder pads is formed on a printed circuit board (PCB) on which the packaged integrated circuit is to be mounted and connected. Solder balls are formed on the solder pads of either the package or the printed circuit board (PCB), and then heated to a temperature sufficient to melt (reflow) the solder balls so as to solder the package to the printed circuit board (PCB).
Any BGA package that utilizes solder balls to electrically couple the packaged integrated circuit to a main PCB is susceptible to failure, particularly fatigue failure, due to temperature fluctuations and dissimilarity in thermal expansion rates for the components that form the BGA package. Specifically, as the BGA package cools down from the solder reflow temperature or undergoes ambient temperature changes during use, the BGA package experiences thermal deformation of different rates in different portions, thereby stressing and straining solder balls. The stresses are concentrated at the interfaces between the solder balls and the solder pads. Especially, as the pin count grows, the distance between the center and the farthest solder ball of the package, the “distance to the neutral point” (DNP), grows longer and thus the stresses on these outer solder balls cause cracking during thermal cycling.
Furthermore, electrical current flowing through the BGA package during operation causes the BGA package to cyclically or intermittently heat up and cool down, which results in additional thermal expansion and contraction, nevertheless such thermal expansion and contraction is considerably less than those at reflow. The stresses from the cyclical or intermittent heating and cooling of the BGA package during normal operation may lead to fatigue failure of the BGA package if the stresses are not properly released.